Seasonal variations and biological regulation of deep-sea CO2 species in the Eastern Indian Ocean

Abstract

The ocean absorbs anthropogenic CO₂ and sequesters it in deep water through the marine biological pump, playing an irreplaceable role in global climate change. This study investigates the vertical distribution of CO₂ system parameters in the deep Eastern Indian Ocean during autumn 2020 and spring 2021. Additionally, the role of biological processes in regulating the CO₂ system parameters were examined via incubation experiments. Results showed that dissolved inorganic carbon (DIC) and alkalinity concentrations were higher in deep layers compared to those in upper layers. pH decreased initially and then increased with depth, while the partial pressure of CO₂ followed an opposite trend to pH in both seasons. Overall, DIC concentration was higher, while dissolved organic carbon concentration and δ¹³C-DIC value were lower in the water column during spring 2021 than autumn 2020, indicating strong respiration during spring 2021. The surface seawater DIC absorption rates during autumn 2020 followed the sequence: St. 1 > St. 4 > St. 2, while the Chl-a increasing rates were: St. 1 > St. 2 > St. 4. The discrepancy indicated the relatively low production at St. 2 and high photosynthetic efficiency at St. 4, which was dependent on the light, phytoplankton composition and abundance. The DIC production rates declined with depth during spring 2021, which was controlled by temperature. The integrated-average rates in the water column followed the sequence: St. 8 > St. 6 > St. 5 > St. 7, heavily relying on microorganism species and abundance, which revealed that respiration processes in the aphotic layer also played important roles in regulating DIC variations and the carbon cycling in the ocean.